baffle step follows the transition from 2pi to 4pi space.
this can be seen with this curve (which is a 24" sphere "baffle"):
from here:
http://www.trueaudio.com/st_diff1.htm
the -3db down point (the half way point) can be calculated using this formula:
from:
http://www.quarter-wave.com/General/BSC_Sizing.pdf
so in a typical room, it won't be the full 6db because real rooms aren't 4pi space beyond the baffle (the walls /ceiling/floor enclose some of the sound).
inputting 24" into the equation, gives a -3db point of 190hz, which matches up with the curve above.
so if you want to push the curve fall enough down to effectively have full loading of the driver in 2pi space at about 80hz, the baffle would probably have to have a midpoint of 40hz or even 20hz, which implies a baffle width of about 10-20 feet or effectively an entire baffle wall for a home environment.
now there is another possible option if you have front corners of the room open.
and that is to follow something like cowan did in his build.
http://www.cowanaudio.com/finale.html
that eliminates all the cancellations that normally create problems behind the speakers and give toe-in for good sound across the entire room.